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Sodium acetate is an ionic compound. The main difference between it and the ionic compounds we’ve seen so far in class is that the anion isn’t monoatomic; it’s polyatomic: Like the other ionic compounds we’ve seen in class, sodium acetate forms a lattice. An aqueous solution of sodium acetate will form a lattice incorporating three water molecules per formula unit of sodium acetate. This gives an overall formula of Na(CH 3 CO 2 )  3H 2 O, called sodium acetate trihydrate. So, if a solution was prepared from a 1 : 3 mixture of sodium acetate : water, the whole solution would freeze as a single lattice (looking more like ice than like a precipitate). Sodium Acetate Heat Pads (has video)

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Sodium acetate trihydrate has a freezing point (or melting point) of 58  C – a temperature similar to that of hot water from a tap. Sodium acetate trihydrate also has the interesting property that it can be supercooled – cooled below its freezing point without actually freezing. This is the basis of sodium acetate heating pads: A supercooled solution of sodium acetate in water is sealed inside a plastic bag containing a metal disk. The metal disk can be pressed to release a tiny crystal of sodium acetate which instantly provides a “nucleation site” so that the rest of the solution solidifies into a lattice. Heat is released as the cations and anions come together to form an ordered ionic lattice (enthalpy of lattice formation!), warming the heat pad up to about 58  C. The heat pad can be “regenerated” by heating it enough that the sodium acetate trihydrate melts then allowing it to slowly cool (without pressing the disk) without freezing. Sodium Acetate Heat Pads (has video)